Already for several decades, use has been made of corrosion inhibitors which tend to evaporate or sublimate even under normal conditions and thus can pass via the gas phase onto metal surfaces that are to be protected, for the temporary corrosion protection of metal objects within closed spaces, e.g. in packagings, switch cabinets or display cases. This manner of protecting metal parts against corrosion during storage and transport is the clean alternative to temporary corrosion protection using oils, greases or waxes and is becoming more important with the increasing globalization of national economies.
It is known that all measures of temporary corrosion protection for protecting metals against the effects of neutral aqueous media or condensed water films have the aim of preserving the primary oxide layer (POL), which is always present on utility metals after first contact with the atmosphere, against chemical and mechanical degradation (cf. e.g.: E. Kunze (ed.), Korrosion und Korrosionsschutz, Vol. 3, Wiley-VCH, Berlin, Weinheim, New York 2001, page 1680 ff.).
This is because the initial reactions of the corrosion of metals (Me) in aqueous media always consist of a selective or general degradation of the respective POL, generally formulated as follows for example:MeO+2H2O→Me(OH)2→Me2++2OH−  (1)before the exposed metal can be oxidized in subsequent steps, e.g.:Me→Me2++2e−  (2)½O2+H2O+2e−→2OH−  (3)Me+½O2+H2O→Me2++2OH−  (4)
One possibility for delaying these sub-steps to a greater or lesser extent by volatile corrosion inhibitors (VCI) and thus achieving a temporary corrosion protection consists in using amines.
Amines as organic derivatives of ammonia NH3 react as a base during hydrolysis, e.g.:NH3+H2ONH4++OH−  (5)thereby producing, in addition to the hydroxyl ions OH−, also ions of the generally formulated type NRH3+, NR2H2+ or NR3H+, depending on whether primary (NRH2), secondary (NR2H) or tertiary (NR3) amines are used.
With this additional formation of OH− ions, at least two effects can be achieved:                a limiting of the tendency of the POL to disintegrate (suppressing of reaction (1)) and        a hindering of the oxygen reduction after reaction (3) as a cathodic sub-step of the corrosion reaction (4).        
Since many amines already have a vapor pressure or sublimation pressure under normal conditions, their use as VCIs is obvious and is described in many patents. Mention is made primarily therein of the cyclic amines dicyclohexylamine and cyclohexylamine. In the U.S. Pat. Nos. 600,328, 2,419,327, 2,432,839, 4,051,066, 4,275,835, DD 284 254 and DD 284 255 cited by way of example, account is already taken of the fact that no reliable temporary corrosion protection can be achieved with amines alone, and therefore the use of amines is combined with substances capable of acting as passivators. As a result, it is possible to recreate the POL spontaneously as an oxidic top layer on metal substances when it has been destroyed by partial chemical disintegration or local mechanical removal (abrasion, erosion) (cf. e.g.: E. Kunze, loc. cit.; E. Vuorinen, E. Kalman, W. Focke, Introduction to vapour phase corrosion inhibitors in metal packaging, Surface Engineering, Vol. 20 (2004) 281ff.)).
As such passivating oxidation agents, the nitrites as salts of nitrous acid have proven useful in practical corrosion protection. They have therefore also been used for a long time as VCIs. In particular, the relatively readily volatile dicyclohexylammonium nitrite has already been used as a VCI for more than 60 years (cf. e.g. Kunze, loc. cit.; Vuorinen et al, loc. cit.) and is mentioned as a component of VCI compositions in numerous patents (e.g.: U.S. Pat. Nos. 2,419,327, 2,432,839, 2,432,840, 2,534,201, 4,290,912, 4,973,448, JP 02085380, JP 62109987, JP 63210285 A, DE 4040586).
The effect of the nitrite ion as an oxidation agent is associated with its electrochemical reduction, e.g. as follows:2NO2−+2H++2e−→2NO+2OH−  (6)
Since hydroxyl ions, OH−, are produced in the process, the reduction in aqueous media proceeds less intensively the higher already the pH of this medium.
With regard to this aspect, it is disadvantageous that values around pH≈9 are set in water at room temperature due to the dicyclohexylamine or the dicyclohexylammonium ions produced as a result of dissociation of the dicyclohexylammonium nitrite. This is moreover not only a hindrance to the development of the passivator effect of the nitrite but also places at risk for example the stability of the passive oxide layer of zinc and aluminum materials. It is known that the oxides of these metals are resistant only in the neutral range and disintegrate increasingly at pH>8 with formation of zincate or aluminate.
With the aim of creating VCI packaging materials which can be used not only for ferrous metals but rather at least also for galvanized steels and aluminum materials, attempts have been made to formulate VCI combinations which contain not only amine nitrites but also components which have a pH-regulating effect in condensed water films on metal surfaces, so that no disintegration of passive oxide layers takes place.
It has therefore been proposed to combine nitrite amine mixtures with other substances capable of sublimating, such as for example the salts of medium-to-weak, saturated or unsaturated carboxylic acids, cf. e.g. U.S. Pat. Nos. 2,419,327, 2,432,839, 2,432,840, DE 814 725. As a result, an improved protection of customary Al and Zn materials is achieved when these are in contact with an aqueous medium or condensed water film; at the same time, however, the passivator properties of the nitrite are reduced by this species.
It is known that the carboxylates in question build up pH buffer systems with a relatively high buffer capacity in aqueous media or condensed water films on metal surfaces with or without the simultaneous presence of an amine, depending on the respectively present carboxylic acid/salt system, and thus hinder the reducibility of oxidation agents, which can be seen in principle from the above reduction reaction for nitrite (6). As is known, this reaction which is necessary for the passivation effect proceeds freely from left to right only if the respective reaction medium does not already have a correspondingly high concentration of OH ions or the resulting OH− ions are regularly discharged from the medium.
If these conditions do not exist, then the passivation effect can be achieved only if the concentration of the oxidation agent in the medium is set in comparative terms to be much higher than the OH− ions produced, for example by continuously supplementing reacted quantities of the oxidation agent from a depot.
All the inventions which propose VCI combinations containing, in addition to an oxidation agent such as nitrite, chromate or an organic nitro compound, also an amine or amine carboxylate can therefore be successful for practical use only if the passivating oxidation agent is used in excessive concentrations relative to the quantity of the other active substances. However, this fact is not always readily clear from the patents in question, since the concentration ranges in which the VCI combinations according to the invention can be used are usually specified in very broad terms. Such VCI combinations containing oxidation agents are described e.g. in U.S. Pat. No. 600,328, where it is recommended to use as much of an organic nitrite salt as possible, and in DE 814 725, in which nitrite salts of organic nitrogen-containing bases (e.g.: carboxylates, piperidines, oxazines or morpholines) are proposed on condition that at least 0.5 to 20 g of the nitrite are to be applied per m2 of packaging material, and reliable protection is achieved only once at least 35 to 600 g have been emitted per m3 of internal space of the packaging.
Since nowadays the practical use of said oxidation agents is regulated due to their known more or less damaging effect on humans and the environment, and since there are occupational exposure limits (OELs) which must be adhered to with regard to the concentration in preparations (cf. e.g. classification of substances and preparations according to EC Directive 67/548/EEC including annual updates), said VCI combinations containing excessive quantities of passivator can no longer be used.
As a replacement for these, it has been proposed for example in U.S. Pat. Nos. 5,209,869, 5,332,525 and EP 0 662 527 A1 to combine the VCI mixtures consisting of nitrites and amine carboxylates with or without molybdate with a desiccant such as silica gel, so that the formation of a condensed water film on the metal surface to be protected and the associated disadvantageous pH effect are delayed for as long as possible. However, this proposal has the significant disadvantage that the VCI system fixed on or in the packaging material tends to absorb considerable amounts of water from the environment due to the presence of the desiccant, which in turn leads to an impairment of the emission rate of the VCI components in the interior of closed packages and thus to a reduction of the VCI corrosion protection effect.
Most of the VCI systems known to date, which contain simultaneously a nitrite and an amine, are unable to provide the necessary reliability for the reasons already mentioned above. Another factor of uncertainty has in the meantime proven to be the fact that particularly the secondary amines and cyclic nitrogen-containing compounds, such as e.g. morpholine and piperidine, which are introduced as VCI components are easily converted to N-nitroso compounds. These N-nitrosamines usually act as weak oxidation agents and promote the corrosion of the metals. Much more disadvantageous, however, is their carcinogenic effect, which prevents these VCI systems from being used on an industrial scale.
It was first attempted to overcome this disadvantage by replacing the nitrite with another oxidation agent, since it was to be presumed that the nitrosation of the amines is caused only by the simultaneous presence of nitrite. In U.S. Pat. No. 4,051,066, therefore, m-nitro- and dinitrobenzoate are used instead of the nitrite, whereas DD 268 978 and DD 295 668 propose the use of dicyclohexylamine-o-nitrophenolate and dicyclohexylamine-m-nitrobenzoate. Finally, U.S. Pat. No. 1,224,500 generalizes regarding the use of volatile aliphatic and aromatic nitro compounds together with heterocyclic amines and mentions 2-nitropropane, nitrobenzene and dinitrobenzene specifically.
On the one hand, however, the passivator properties of these alternative oxidation agents proved to be much weaker than those of nitrite, and on the other hand the intended effect was not achieved, i.e. avoiding the formation of N-nitrosamine with the amines used. In the meantime it has become known that such well-proven VCI components, such as morpholine and dicyclohexylamine, undergo nitrosation simply due to the normal constituents of air, in particular upon contact with metals and at relatively high temperatures. In practice, this prevents them from being incorporated in plastics since, as is known, melt extrusion, injection molding or extrusion blow molding takes place at temperatures around 200° C. in metal machines.
In order to satisfy the demand for films and hard plastics equipped with VCIs to cope with overseas transport, the use of amine-free, nitrite-containing VCI systems has been proposed. For example, U.S. Pat. No. 3,836,077 mentions the combination of nitrite with borate and a phenol which is mono-, di- or trisubstituted with styrene. However, the VCI corrosion protection effect remains minimal since neither the borate nor the aromatically substituted phenols sublimate out of the polymeric carrier materials.
U.S. Pat. No. 4,290,912, on the other hand, emphasizes the use of inorganic nitrites in combination with a trisubstituted phenol and silica gel for the production of VCI films, but the examples of embodiments show that, in the case of phenols, only aliphatically substituted phenols and especially 2,6-ditert-butyl-4-methylphenol (butylated hydroxytoluene, BHT) are meant. Since these substituted phenols tend to sublimate even at normal temperature, an improved sublimation rate was able to be achieved with this combination for nitrite without the involvement of a volatile amine, but the nitrite reaching the metal surface cannot provide reliable VCI corrosion protection without the use of further components. In the case of passivating metals, it is known to be necessary to also use components which adjust the pH in condensed water films in a range which is favorable for passivation and which stabilize the resulting passive oxide layer by adsorption to prevent disintegration (cf. e.g. E. Kunze, loc. cit.), something which is not achieved with the active substance combinations claimed in U.S. Pat. No. 4,290,912. Furthermore, in the case of copper materials in neutral aqueous solutions, the nitrite causes blackening due to the formation of the oxide CuO.
Benzotriazole has long been used specifically for protecting copper and copper alloys against atmospheric corrosion (cf. e.g. Kunze, loc. cit.). However, since the tendency of this compound to sublimate is relatively low, it is proposed in DE 1182503 and U.S. Pat. No. 3,295,917 firstly to set the depot of this VCI to a higher temperature (up to approx. 85° C.) and at the same time to cool the metal objects on which the condensation is to take place. On the other hand, U.S. Pat. Nos. 2,941,953 and 3,887,481 describe the impregnation of paper with benzotriazole and/or tolyltriazole. Organic solvents such as tetrachloroethylene are used, and it is specified that the metal parts to be protected should be wrapped as closely and as tightly as possible with the VCI packaging material thus impregnated, in order to keep as small as possible the distance between the VCI depot and the metal surface to be protected. However, this technology has the disadvantage that the active substance in the form of extremely fine powder particles adheres to the paper only slightly and can easily slip off, so that the corrosion protection properties of this packaging material cannot be reliably configured. Furthermore, they would remain limited exclusively to copper materials.
In order to provide VCI-emitting packaging materials which can be used for the corrosion protection of various metals, a wide range of combinations of active substances have already been proposed. In this regard, EP 0662527 mentions mixtures of benzotriazole with cyclohexylamine benzoate and ethylamine benzoate or with anhydrous sodium molybdate and dicyclohexylamine nitrite, U.S. Pat. No. 4,051,066 and UD 4,275,835 mention mixtures of benzotriazole with ammonium molybdate and amine molybdates, amine benzoates and amine nitrates, U.S. Pat. No. 4,973,448 mentions mixtures of benzotriazole with organic carbonates, phosphates and amines, and finally JP 62063686 and 63210285 A mention mixtures of benzotrizaole with alkali and amine salts of aromatic carboxylic acids.
Combinations of benzotriazole, tolyltriazole or methylbenzotriazole with other nitrogen-organic volatile solids are described e.g. in JP 62109987, JP 61015988, DD 268978 and DD 298662. It is disadvantageous that all the ammonium ion-containing components and amine-containing components, due to their more or less pronounced tendency to form complexes with metal ions, reduce the protective effect of triazoles, in particular with regard to nonferrous metals. In addition, said amines and ammonium compounds are highly hydrophilic. VCI depots which contain such substances tend to absorb more water. Their hydrolysis then usually leads to a greater reduction in their tendency to sublimate, which inevitably results in a reduction in the corrosion protection effect.
In order to benefit from the advantages of using VCIs and the inhibitor effect of the triazole structure, it is proposed in JP 03079781 to use only alkylaminotriazoles instead of the triazole/amine substance combinations. In fact, the explicitly mentioned substances 3-amino-1,2,4-triazole and 3-amino-5-methyl-1,2,4-triazole have a higher rate of volatilization, but do not have such a clear corrosion protection effect as benzotriazole and tolyltriazole, in particular with respect to copper.
Moreover, the proposed alkylaminotriazoles would in any case not be suitable for use alone as corrosion inhibitors for the wide range of utility metals.
VCI-emitting packaging materials, which are said to be suitable for the temporary corrosion protection of both ferrous and nonferrous metals, consist according to DE 101371130 and U.S. Pat. No. 6,752,934 B2 of substance combinations which contain, in addition to a nitrite, also water-insoluble, polysubstituted phenols, aliphatic esters of a dihydroxybenzoic acid, and tocopherol (2,5,7,8-tetramethyl-2-(4′,8′,12′-trimethyltridecyl) chroman-6-ol. With the organic components of these combinations, the largest surface region of the metal parts to be protected is coated with a hydrophobicizing adsorption film, so that the passivating effect of the nitrite has to be provided only on the few surface regions of the metal parts on which no adsorption took place.
Since both polysubstituted phenols and also tocopherols can act as antioxidants, Cu and silver base materials within the packaging materials from which such a substance combination is emitted moreover remain free of black or dark gray tarnish films. However, one condition for this is on the one hand that the surfaces of the metal parts to be protected are in a dry, hydrophobicizable condition at the time of packing and a relative humidity which is as low as possible (≦60% at 20° C.) prevails in the packing area. Furthermore, it must be ensured that not only the nitrite but also the antioxidants sublimate out of the respective packaging material and are adsorbed as a thin film onto the metal surfaces to be protected. Although DE 101371130 provides for this reason that a bicyclic terpene or aliphatically substituted naphthalene is added as a further component, which is intended to help said VCI components to always be emitted to a sufficient degree even at relatively low temperatures, account must be taken of the fact that this is not done in all cases. Therefore, failures of the VCI corrosion protection when these packaging materials are used in areas with higher relative humidities and in the case of Cu base materials are not ruled out.